This invention relates to circuits for compensating for voltage variations in a supply network, and more particularly, to a circuit for maintaining constant a voltage across two conductors of an A-C supply network which supplies a load having a rapidly changing impedance.
A circuit which is commercially used to control the voltage of a supply network which supplies electric power to furnaces which are used in the manufacture of steel and the melting of scrap is described in "Siemens-Forschungs-und Entwicklungs-Bericht", Vol. 6 (1977), pp. 29 to 38. In an electric furnace for making steel or melting scrap, an electric arc which is produced between the electrodes of the circuit and the material to be melted is randomly interrupted as the material melts. Such widely varying load impedances are also found in rolling mills which contain pulse power supplies for operating synchrotrons or converter drives. The rapid and wide excursions in the amplitude of the voltage and current can create problems to other consumers of electric power on the same network. Although a supply network may have an impedance which has a negligible resistive component, such networks may have large reactive impedances which produce large reactive currents in response to the voltage variations. For example, other electric power consumers which employ incandescent lamps connected to the supply network will be subjected to annoying fluctuations in brightness. It is necessary to suppress the effects of such load variations and the consequential reactive currents because most such loads cause the incandescent lamps to flicker in the frequency range of 3 to 10 Hz, and at amplitudes of 0.5%, which are in the ranges perceivable by the human eye.
The circuit described in the above publication is provided with a battery of capacitors connected in shunt with a load which is connected to a three-phase supply network, the capacitors being capable of providing as much reactive current as the load may maximally consume. The circuit is further provided with a three-phase control element having electric valves which are connected to the supply network and which are fired by means of a .DELTA. circuit. The three-phase control element consists of a choke connected in series with an A-C control element having two controlled switching valves which are connected in parallel to one another but poled for conduction in opposite directions. The valves are controlled by circuitry responsive to the current flowing through the load as well as the current flowing through the three-phase control elements. Such circuitry consists of a multiplicity of computing elements which perform the functions of coordinate transformation, vector identification and vector rotation. Such a system is expensive and complex.
It is, therefore, an object of this invention to provide a simple and fast acting control circuit which maintains the voltage level between the individual conductors of a single or multiphase supply network constant for at least a short time.
It is another object of this invention to maintain the voltage level between the conductors of a transmission network, illustratively between each phase conductor and a neutral conductor, or between two phases of a polyphase network, constant at a predeterminable mean value.
It is a still further object of this invention to maintain the voltage between the conductors in a transmission network constant at a predeterminable value responsive to a predetermined function, which may be a power of the voltage, so as to influence an RMS value of the voltage.
It is another object of the invention to selectively maintain the voltage between the conductors of a transmission network constant or permit such voltage to vary over a predetermined period of time of sufficient duration that the variations will not adversely affect other consumers.